Increasing paternal birth weight has been associated with increased risk of fathering a preterm infant, causing speculation that a fetus programmed to grow rapidly can trigger preterm labor. [unreadable] In order to evaluate this possibility, pregnancies occurring from 1974 to 1989 among women themselves born in the Danish Perinatal Study (1959-61) were identified through the Danish Population Register. This register lists all births in the country along with the Personal Identification Number (akin to the U.S. Social Security Number) of the parents. We used this information to locate the obstetrical records of these 1974-89 births, which were then abstracted. Paternal birth weight was obtained by linking Personal Identification Numbers of the fathers to archived midwifery records.[unreadable] After doing this, we found that paternal birth weight was not associated with the probability of fathering a preterm infant overall. However, there was a significant interaction between paternal and maternal birth weights (p0.003). When the mother weighed less than 3 kg at birth, increasing paternal birth weight was associated with increased occurrence of preterm birth (p for trend0.02); paternal birth weight was unassociated with preterm birth for mothers weighing 3 kg or more at birth (p0.34). Similar results were found for mean infant gestational age, and when only spontaneous deliveries were studied. We concluded that when the mother was born small, increasing paternal birth weight was associated with increased risk of preterm birth, suggesting that a fetus growing faster than its mother can accommodate might trigger preterm birth.